PARENT SESSION
Poster Session 1: Photosynthesis and Water Relations
Monday, August 8, 5:00 PM - 6:30 PM, Exhibit Hall 220 A-E, Level 2, Palais des congrès de Montréal

Water uptake profiles and competition status of eight Northern Rocky conifer species.

Hall, Kathryn^{*,1, 2}, Marshall, John², Duursma, Remko ³, ¹ Environmental Science Program, Moscow, ID, USA² Department of Forest Resources, Moscow, ID, USA³ Department of Forest Ecology, Helsinki, Finland

ABSTRACT- Water uptake depths reflect the type of precipitation used by tree species and provide an index for competition between root systems of neighboring trees. Knowledge of seasonal water sources for a species is immensely valuable for modeling the ecological responses of forests to climate change. Short supply of water during drought years increases competition between species rooted at the same depth. Current forest growth models ignore water uptake depths and belowground competition, limiting the accuracy of these models during aberrant weather years. Objectives of this study were to determine the water uptake depth, precipitation source of that water and competition status of eight co-occurring conifer species during drought conditions. To examine the water uptake profiles of these species, we collected soil and tree core samples at the Priest River Experimental Forest in northern Idaho. Soil samples were collected from 38 soil pits in increments of 10 cm, including the organic layer, to a depth of one meter. Six trees per soil pit were sampled using an increment corer. Soil and tree samples were analyzed for ¹⁸O ratios using a direct equilibration method. The soil ¹⁸O values were modeled using a non-linear mixed effects model with depth as the predictor variable. The model was then inverted to determine the water uptake depths of the trees sampled. Our results suggest that different tree species take up water from a gradient of depths. Some species take up exclusively rainwater at soil depths less than 60cm (T. plicata, T. heterophilia, L. occidentalis) and others tap into very deep ground water layers below 3-4m (P. ponderosa), confirming previous studies. Other species (P. contorta, P. monticola, Pseudotsuga menziesii, Abies grandis) fell in between these two groups implying possible competition. Our findings will be valuable for applications including forest growth modeling and carbon budgeting. Additionally, by knowing which species depend upon snow or rainwater, we will be better equipped to predict ecological responses to changes in climate patterns.

Key words: conifer rooting depths, tree water sources, direct equilibration, water competition